Evaluation of <em>In Vitro</em> Antibacterial Activity of<em> Liquidambar orientalis </em>Mill. Essential Oil Against Clinically Important Bacterial Pathogens

Background: Antibiotic resistance is one of the most critical challenges in treating bacterial infections today. The widespread and often inappropriate use of antibiotics has accelerated the emergence of resistant bacterial strains, making many conventional therapies ineffective. Without effective strategies to address this growing issue, bacterial pandemics that pose severe threats to global public health could emerge in the future. Objectives: This aim of this study was to investigate in vitro antibacterial activity of Liquidambar orientalis Mill. essential oil against strains of Escherichia coli, Staphylococcus aureus, Enterococcus faecalis, E. faecium, Klebsiella pneumoniae, Acinetobacter baumannii, Listeria monocytogenes, Salmonella Typhimurium, Pseudomonas aeruginosa, Mannheimia haemolytica, Brucella melitensis, and Corynebacterium pseudotuberculosis. Methods: The volatile constituents of L. orientalis Mill. essential oil were analyzed by gas chromatography–mass spectrometry (GC-MS), and its antibacterial activity was assessed using disc diffusion and agar well diffusion methods. The McNemar test was used to statistically evaluate the difference in the detection rates of resistance/sensitivity between disc diffusion test and well diffusion test results. Results: GC-MS analysis identified 21 chemical constituents in L. orientalis Mill. essential oil, with cinnamyl cinnamate (62.56%), (1-methylcyclobutyl) benzene (31.30%), and squalene (2.96%) as the major components. Minor constituents such as α-pinene, camphene, benzaldehyde, and benzyl alcohol were detected at concentrations below 1%. The essential oil demonstrated in vitro antibacterial activity against the tested bacterial strains, with inhibition rates of 58.3% and 62.5% in the disc diffusion and agar well diffusion tests, respectively. Antibacterial activity was slightly higher against Gram-positive bacteria (80.0%) compared to Gram-negative strains (78.57%). The highest activity was observed against S. aureus in the disc diffusion test and against B. melitensis in the agar well diffusion test. No statistically significant difference was observed between the two methods regarding resistant/sensitive detection rates. Conclusions: L. orientalis Mill. essential oil exhibited strong in vitro antibacterial activity against S. aureus, B. melitensis, E. coli, E. faecium, K. pneumoniae, and M. hemolytica, particularly at concentrations of 62.5, 31.25, and 15.62 µg/ml. These results suggest its potential as a broad-spectrum antibacterial agent. Notably, when combined with antibiotics, it may offer therapeutic advantages against B. melitensis, the primary causative agent of human brucellosis. However, further in vivo studies are required to validate these findings and assess its practical applications in treating bacterial infections in both human and veterinary medicine.